Americans are fixated these days on polygamy, and it’s fair to say we don’t know how to feel about it. Polygamy evokes both fascination and revulsion — the former when actress Chloe Sevigny is involved, and the latter when it is practiced by patently evil men like Osama Bin Laden and Warren Jeffs, the fundamentalist Mormon leader who had a thing for underage wives.

At the same time, the practice of plural marriage is so outside mainstream Western culture, so far in the past for many Westerners, that it has come to be regarded as almost quaint. What’s so wrong with it, if it “works” for some people? In counterculture circles, the practice of polyamory, or open partnerships, is supposed to be having some sort of moment. All of which explains why, in response to the argument by conservatives like GOP presidential candidate Rick Santorum and U.S. Supreme Court Justice Antonin Scalia that gay marriage could be a slippery slope leading to polygamy, some feminists, lefties and libertarians have wondered aloud whetherpluralmarriage is really so bad.

History suggests that it is. A new study out of the University of British Columbia (UBC) documents how societies have systematically evolved away from polygamy because of the social problems it causes. The Canadian researchers are really talking about polygyny, which is the term for one man with multiple wives, and which is by far the most common expression of polygamy. Women are usually thought of as the primary victims of polygynous marriages, but as cultural anthropologist Joe Henrich documents, the institution also causes problems for the young, low-status males denied wives by older, wealthy men who have hoarded all the women. And those young men create problems for everybody.

“Monogamous marriage reduces crime,” Henrich and colleagues write, pulling together studies showing that polygynous societies create large numbers of unmarried men, whose presence is correlated with increased rates of rape, theft, murder and substance abuse. According to Henrich, the problem with unmarried men appears to come primarily from their lack of investment in family life and in children. Young men without futures tend to engage in riskier behaviors because they have less to lose. And, too, they may engage in certain crimes to get wives — stealing to amass enough wealth to attract women, or kidnapping other men’s wives.

As marriage historian Stephanie Coontz has pointed out, polygyny is less about sex than it is about power. Rich old guys with lots of wives win twice: They have more women to bear them babies and do household work, and they also gain an advantage over other men. After all, in such societies a young man in want of a wife cannot simply woo her. There is too much competition, and he probably has too little to offer. So he winds up having to do work for a more powerful, polygynous man, bringing him gifts and tributes, in hopes of someday being rewarded with one of that man’s many daughters. “Often the subordination of women is in fact also a way of controlling men,” says Coontz, who was not involved in the study out of UBC.

That polygyny is bad for women is not necessarily intuitive. As economist Robert H. Frank has pointed out, women in polygynist marriages should have more power because they’re in greater demand, and men should wind up changing more diapers. But historically, polygamy has proved to be yet another setup that screws the XX set. Because there are never enough of them to go around, they wind up being married off younger. Brothers and fathers, realizing how valuable their female relations are, tend to control them more. And, as one would expect, polygynous households foster jealousy and conflict among co-wives. Ethnographic surveys of 69 polygamous cultures, “reveals no case where co-wife relations could be described as harmonious,” Henrich writes, with what must be a good dose of understatement.

Children, too, appear to suffer in polygamous cultures. Henrich examines a study comparing 19th-century Mormon households, 45 of them headed by wealthy men, generally with multiple wives, and 45 headed by poorer men, generally with one wife each. What’s surprising is that the children of the poorer men actually fared better, proving more likely to survive to age 15. Granted, this is a small study, but it’s consistent with other studies, including one from Africa showing that the children of monogamous households tend to do better than those from polygynous households in the same communities. Why? Some scholars suspect that polygyny may discourage paternal investment. Men with lots of children and wives are spread too thin, and to make things worse, they’re compiling resources to attract their next wives instead of using it on their existing families.

Must polygamy always bring these social ills? Is it possible to be polygamous in a way that’s good for you and everyone else? Maybe. Historically, problems have cropped up when polygamy is widespread in a culture with great disparities in wealth, and a few men hoard all the women. But it has worked in small cultures where there aren’t a lot of differences in wealth and status. Coontz points to past Native American societies that occasionally engaged in what’s known as sororal polygyny, in which a man married to one woman might also marry her sister, perhaps after the sister’s husband died.

It’s possible that even in a large, deeply stratified society like ours, rare instances of polygamy wouldn’t foster gender inequity and roving bands of unhappy single men, provided those instances were spread out among a largely monogamous population. But it’s hard to imagine that, because it isn’t how it has played out here. Instead, American polygamy occurs in close-knit fundamentalist Mormon communities, in which young women often do appear to be subordinated and from which young men — the so-called “lost boys” — are exiled to reduce the competition for wives. Has fundamentalist Mormon culture shaped the expression of polygamy, or has widespread polygamy shaped fundamentalist Mormon culture? It’s hard to separate the two.

And this is exactly Henrich’s point: Polygamy may actually exacerbate inequities in wealth and gender that hurt societies, even if the institution itself appears neutral. Crime and chaos are threatening. Christianity may have brought monogamy to Europe and many other places, but those cultures succeeded because monogamy happened to suit them. In other words, as far as social evolution is concerned, the best form of marriage for a given society isn’t really about what’s moral, but what works.